The present invention relates to radio receiver devices, and more particularly to techniques for correcting for inaccuracies in in-phase/quadrature-phase (I/Q) demodulators.
As shown in FIG. 1, low-cost receivers 10 for wireless digital communications systems typically use an analog I/Q demodulator 20 to downconvert received RF signals to baseband before the signal is converted to digital data by an analog-to-digital converter (ADC) 30 and is processed digitally. An example of an I/Q demodulator 20 is shown in FIG. 2 and comprises an in-phase mixer 22 and a quadrature-phase mixer 24, and associated low pass filters 26 and 28. An I/Q demodulator never exhibits perfect quadrature and amplitude balance between the I and Q paths, causing image artifacts to appear at the negative of the input signal frequency as shown in FIG. 2.
State-of-the-art I/Q demodulator implementations typically provide an image suppression of 35–40 dB, which is sufficient to guarantee acceptable performance in most applications. However, for real-time spectrum management applications where it is desired to detect the types of signals occurring in a shared frequency band, the required image suppression is much larger than 35 dB. In fact, in some cases an image suppression of approximately 90 dB is required. Image artifacts above the noise floor make spectrum analysis displays confusing. In addition, signal classification and pulse detection functions can be confused by the image artifacts if they are not properly removed.
There are several known techniques for mitigating the image artifacts inherently created by non-ideal I/Q demodulators. According to one technique, an intermediate frequency (IF)-based receiver is used and digital I/Q demodulation is performed at IF. The drawback of this technique is that IF receivers can only be implemented using discrete analog circuits. Another technique involves using calibration to measure the amplitude and phase imbalance of the I/Q modulator and correct for the imbalances digitally. The problems with this technique are that it can only provide up to 65 dB of suppression over temperature, and a relatively long calibration time is required at manufacture.
Still another approach is to use a complex very low intermediate frequency (VLIF)-based receiver. With this approach, however, out-of-band signals can appear as if they were in-band, still requiring calibration to minimize this effect.
An improved approach for removing the effects of I/Q demodulator imbalance is needed that is efficient and flexible to implement, and capable of providing significant image suppression required for certain communication applications, such as spectrum analysis or management applications.